Causes of settlement |
Value of angular distorsion |
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cause |
comment |
δ : settlement |
Θ: radius of settlement |
H: heigh of building |
Length basis of building |
compression of foundation soils under static loads |
soft normally consolidated clays and peaty soils are most compressible. loose |
Type of building |
L / H |
allowable |
|
Compression of soft clays due to lowering ground water table |
Increased effective stress causes settlement with no increase in surface load |
steel frame with flexible siding |
- |
0.008 |
|
steel or reinforced concrete frame with intensive finish such as dry wall, glass or moveable panels |
- |
0.002 to 0.003 |
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Compression of cohesion less soils due to vibrations |
Loose sands and gravels are most susceptible. Settlement can be caused by machine vibrations, earthquakes and blasts |
steel or reinforced concrete frame with brick, block, plaster or stucco finish |
≥ 5 ≤ 3 |
0.002 0.001 |
|
load bearing brick, tile or concrete blocks walls |
≥ 5 ≤ 3 |
0.0008 0.0004 |
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Compression of foundations soil due to wetting |
Loose silty sands and gravels are most susceptible. Settlements can be caused by rise in groundwater table or by infiltration |
circular steel tanks on flexible base with fixed top |
- |
0.008 |
|
circular steel tanks on flexible base with floating top |
- |
0.002 to 0.003 |
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Shrinkage of cohesive soils caused by drying |
highly plastic calys are most susceptible, increase in temperature under buildings containing ovens or furnaces may accelerate drying. Wetting or highly plastic clays can cause swelling and heave of foundations |
tail slender structures, such as stacks, silos, water tanks with rigid mat foundations |
- |
0.002 |
|
Mind |
Foundation Possibilities for Different Subsoil Conditions |
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Loss of foundation support due to erosion |
Waterfront foundations must extend below maximum erosion depth |
regular / irregular settlem. & radius of settled area |
Subsoil conditions |
Foundation Possibilities |
|
Loss of foundation support due to excavation of adjacent ground |
Mos pronounced in soft saturated clays |
|
Light flexible structure |
Heavy rigid structure |
|
Loss of support due to lateral sifting of adjacent ground |
Lateral shifting may result from landslides, slow downhill creep or movement of retaining structures |
|
Deep compact or stiff deposit |
Footing foundations |
1. Footing foundations 2. Shallow mat |
Loss of support due to partial or complete liquefaction |
Loose saturated sands are most susceptible |
|
Deep compressible strata |
1 Footing foundations on compacted granular zone 2. Shallow mat 3. Friction Piles |
1. Deep mat with possible rigid construction in basement 2. Long piles or caissons to by-pass 3. Friction piles |
Loss of support due to formation of skinhole |
Soils overlying cavernous limestone and broken conduits are susceptible |
|
Soft or loose strata overtlying firm strata |
1. Bearing piles or piers 2. Footing foundations on compacted granular zone 3. Shallow mat |
1. Bearing piles or piers 2. Deep mat |
Loss of support due to thawing of permafrost foundation heat |
permafrost should be insulated from heat source |
|
Compact and stiff layer overlying soft deposit |
1. Footing foundations 2. Shallow mat |
1. Deep mat (floating type) 2. Long piles or caisons to by-pass soft deposit |
Down drag on piles driven through soft clay |
Loading on piles is increased by negative skin friction of soil around upper part of pile settles |
|
Alternating soft and stiff layers |
1. Footing foundations 2. Shallow mat |
1. Deep mat 2. Piles or caissons to underlying firm stratum |